1. Field of the Invention
The invention relates generally to the field of solid/liquid separation systems and, more particularly, to a belt press having an elliptically shaped inlet guide or grid which allows the pressure on a solid/liquid mixture in the belt press to be controlled.
2. Technical Considerations
Various systems are known in the art for dewatering solid/liquid mixtures, suspensions or slurries, such as sewage sludge or other flocculated or unflocculated slurries. Such mixtures, suspensions or slurries will be referred to hereinafter simply as "mixtures". In cases where the solids are formed primarily of compressible, granular, fibrous or cellular components, belt presses are typically used to separate the solid components from the liquid components. Belt presses dewater the solid/liquid mixture by applying an increasing surface pressure to the mixture as the mixture is carried between two moving belts and through a series of press rollers.
One example of a prior art belt press is schematically shown and generally designated 10 in FIG. 1 of the drawings. The belt press 10 has a frame 12 with an upper belt 14 and a lower belt 16. The belts 14 and 16 are typically liquid permeable. The upper belt 14 is rotatably mounted around a plurality of guide rollers 18. The lower belt 16 is independently, rotatably mounted around a plurality of guide rollers 18'. The upper and lower belts 14 and 16 are looped around a series of press rollers 20.
To dewater a solid/liquid mixture, the mixture is continuously fed into a feed box 22 located above the upper surface of the lower belt 16. The feed box 22 distributes the mixture uniformly over the top of the lower belt 16. The belts 14 and 16 are rotated in conventional manner, such as by electric motors turning the guide rollers 18, 18'. As the lower belt 16 moves toward the left, as shown in FIG. 1, the bottom of the lower belt 16 rides across a stationary inlet grid 24 fixedly mounted on the frame 12. The inlet grid 24 has a flat or planar entrance portion 25 near the feed box 22 and a short, curved exit portion 26 at the other end. The exit portion 26 has a constant radius.
As the belts 14 and 16 rotate to the left, as shown in FIG. 1, the mixture enters a "wedge zone" 27 between the upper and lower belts 14 and 16 where pressure is applied to the mixture by the belts. As the mixture is pressed in the wedge zone 27 between the two belts 14 and 16, liquid is pressed out of the mixture to start forming a sludge or filter cake between the upper and lower belts 14 and 16. A pressing mechanism 28, such as a plate or a series of rollers, is generally located above the lower portion of the upper belt 14 to press the upper belt 14 toward the lower belt 16 to increase the pressure on the mixture between the belts 14 and 16. The exit portion 26 of the inlet grid curves downwardly to guide the upper and lower belts 14 and 16, along with the filter cake trapped therebetween, around a perforated roller 29 where more liquid is pressed out of the filter cake. The belts 14 and 16, along with the filter cake therebetween, move through the series of press rollers 20. The press rollers 20 are arranged in decreasing diameters in a direction of movement of the belts 14 and 16. The press rollers 20 provide an increasing compressive and shearing force on the filter cake to further dewater the filter cake. At the end of the series of press rollers 20, the belts 14 and 16 separate and the dewatered filter cake is released and collected.
While generally adequate for dewatering sludge, such conventional belt presses 10 have several shortcomings. One problem with the known belt presses 10 is that if the pressure on the solid/liquid mixture between the belts 14 and 16 is increased too quickly, some of the mixture may be ejected sideways from between the belts 14 and 16. Further, since the exit portion 26 is of constant curvature, it provides an increased pressure of about the same magnitude over its entire area. It is, therefore, difficult to gradually increase the pressure on the mixture as it moves through the wedge zone 27. Additionally, the mechanical pressing mechanism 28 must be serviced and maintained to keep it in proper working order. Another problem with such known belt presses is that it is difficult to change the pressure profile on the filter cake to compensate for various viscosities or thicknesses of different materials to be filtered. While some belt presses provide for some adjustment in the wedge zone, this is generally accomplished by changing the convergence angle of the two belts. However, this adjustment typically also changes the pressure rise profile. That is, increasing the angle between the belts causes a more rapid pressure rise which must be accounted for by reducing the belt tension and, hence, the overall performance of the press.
Another known dewatering device is disclosed in U.S. Pat. No. 4,879,034 to Bastgen. This device consists of a frame carrying a supporting plate which is flexible so that the curvature of the supporting plate can be changed. The supporting plate can be formed of a number of separate plate parts which are joined together to form the flexible supporting plate. A series of movable members are attached along the supporting plate so that as the movable members are individually pivoted on the frame, the curvature profile of the supporting plate changes. However, this complex, multi-component plate construction adds to the cost of making, using, and maintaining the device.
Therefore, it is an object of the invention to provide a belt press of reduced complexity but which reduces the problems associated with known belt presses. It is additionally an object of the invention to provide a belt press having a pivotable inlet guide or grid to control the pressure on a solid/liquid mixture between the belts of the press. It is further an object of the invention to provide a belt press having an inlet guide in which the angle between the belts may be adjusted by moving an entrance end of the guide to control a tangent point of the upper belt on the lower belt without changing the curvature profile of the inlet guide. It is also an object of the device to provide a belt press which does not require a conventional mechanical pressing mechanism.